Patent Literature 1 discloses an endoscope system that includes two observation modes. In a first normal observation mode, a normal observation image of a paries illuminated by white light is displayed on a display apparatus. In a second special observation mode, a synthetic image obtained by synthesizing an image obtained by illuminating white light with an image of fluorescence generated from a lesion location due to excitation is displayed. For generating the synthetic image, white light and excitation light are alternately irradiated onto the paries via a light guide of the endoscope system (paragraphs [0003], [0004], etc. in specification).
As shown in FIG. 3 of Patent Literature 1, in a light source part of the endoscope system, a dichroic mirror transmits white light and also reflects laser light output as excitation light from a semiconductor laser is provided. An amount of laser light left unreflected by the dichroic mirror is detected by a photodiode and output to an APC (Automatic Power Control) circuit. With this configuration, it becomes possible to instantaneously and highly precisely feedback-control the optical output amount of the semiconductor laser (paragraphs [0040], [0043], [0044], etc. in specification).